The present invention relates to an aqueous phase inhibitor for quenching free radical polymerization in the aqueous phase and related methods. The invention also relates to a coating for providing protection against polymer growth on walls of polymerization vessels and associated equipment including reactors, and on baffles, blades and shafts of agitators.
Most crosslinked polymers and linear polymers are manufactured by free-radical-initiated chain polymerization. Free radical polymerization is the linking together of monomer units under the influence of a free radical initiator. A free radical initiator is a reactive molecular fragment with an unpaired electron. Such polymerization when carried out in an immiscible liquid is also known as suspension polymerization, pearl polymerization, droplet polymerization, and dispersion polymerization. The polymerization are usually performed with water as the immiscible liquid. The polymeric product is made handleable and manageable by this technique, since the polymer is broken-up into spherical droplets before it is formed. In such a polymerization, the monomer mixture containing an initiator is suspended in an immiscible liquid as droplets. The liquid most used for making a slurry of monomer droplets is water. Water is preferred for dispensing liquid since most organic monomers have a very small solubility in water or, if the monomer is water soluble, can usually be salted out by the addition of an electrolyte. The monomer droplets are converted into beads of polymer by heating the slurry to a temperature where the dissolved initiator thermally decomposes into free radicals at a sensible rate. A sensible rate is achieved at a temperature where the half-life of the dissolved initiator is between from about five to about ten hours. This temperature is maintained until the monomer droplets are transformed into beads of solid polymer with essentially no remaining monomer. A higher temperature step then may be applied to the bead slurry in order to destroy the remaining unreacted initiator.
Since all organic monomers have a small but finite solubility in water, monomer diffuses from the droplets into the water phase. If this molecularly dispersed monomer dissolved in the water phase is not protected against polymer growth, it will polymerize during the process in the water phase by initiation from free radicals that arrive in the water phase from the droplets of monomer or by thermal initiation of the monomer itself. Such polymers are much smaller in size (0.005 to 1.0 micron in diameter) than that being formed in droplets (30 to 3000 microns in diameter), and, therefore, produces a milky white aqueous liquor. This polymer, which is called an emulsion polymer, is an unwanted byproduct of the droplet polymerization process, and converts valuable monomer into waste polymer.
The present invention relates to an aqueous phase inhibitor for quenching free radical polymerization in the aqueous phase. For purposes of this invention the term xe2x80x9cinhibitorxe2x80x9d refers to a molecule that terminates the growth of free radical polymerization by interacting with the radical terminus of the polymer chain so as to remove its energy for continued reaction with monomer, and the term xe2x80x9cquenchxe2x80x9d means to suppress, to stop or to destroy. The inhibitor comprises a free radical quenching agent or moiety having a hydrophilic tail. The term xe2x80x9chydophilic tailxe2x80x9d refers to a molecular fragment that is solvated by water. The hydrophilic tail can be a polyhydric alcohol. The polyhydric alcohol tail can comprise from about three to about one thousand carbon atoms, each of the carbon atoms being attached to hydroxyl group except for the carbon atom attached to the N-hydroxylamine. The hydrophilic tail can also be selected from sugar molecules, dextrans, mositol, sorbitol, polyvinylalcohols, polyethylene oxides, aminoacids, oligomers of N-vinylpyrrolidinone, oligomers of N-vinyl formamide, oligomers of acrylamide, oligomers of methacrylamide, p-benzamide, p-benzenesulfonamide, etc.
In one embodiment of the invention, the free radical agent or moiety is N-hydroxylamines. In another embodiment, the free radical agent or moiety is N-nitrosonamine.
The inhibitor of the present invention can be applied as a concentrate in a liquid medium such as water, alcohol and mixture thereof.
In another embodiment, the aqueous phase inhibitor can be N-hydroxyl-N-methylglucamine and the liquid medium is water. The N-hydroxyl-N-methylglucamine can be from about 15 to about 40% of the solution. In a further embodiment, the aqueous phase inhibitor can be N-nitrous-N-methylglucamine and the liquid medium is water. The N-nitrous-N-methylglucamine can be from about 15 to about 40% of the solution.
Unlike other inhibitors whose hydrophilicity is provided by ion formation, the hydrophilicity of the inhibitors of the present invention is provided by a non-ionic carbon chain carrying polarizable groups that are solvated by water through dipole-dipole interaction and by hydrogen bonding. Consequently, the inhibitors of this invention retain their water solubility throughout the entire aqueous pH range. In suspension polymerization, pH control over the aqueous phase is unnecessary. The hydrophilic tail also makes these inhibitor molecules immiscible within the hydrophobic monomer droplets, and this immiscibility precludes quenching the desired polymer development within the droplets.
In yet another embodiment, the present invention also provides a coating protection against polymer growth. For purposes of this invention, the term xe2x80x9ccoatingxe2x80x9d refers to a covering by filming over the surface of the material so as to change the nature of the surface. Most coatings have polar oxide surface layers that will bind to molecules that are dipoles. The coating comprises a free radical quenching agent having a hydrophilic tail. Since these inhibitor molecules have a hydrophilic tail of multi-carbon chains with polarizable groups, they have interfacial activity and seek the interface between the liquid phase and the solid surfaces of reaction vessels and associated equipment such as reactors and agitators. The hydrophilic tail of the quenching agent have dipole oxygen groups for which the oxides surface of the metals has a propensity to bind and form dipole-dipole interactions. By filming out on the reactor surfaces, they act as polymer growth inhibitors or as a coating on the reactor walls, and baffles, stirring blades, and shafts of agitators. The coating on the surfaces of the reactors and agitators keeps the reactors and agitators clean and operable for long periods of time. The metal of reactor, particularly stainless steel reactor, can act as a template for polymer film growth. For example, as many as 100 batches of beads were manufactured in between kettle cleanings in the suspension polymerization of styrene-divinyl-benzene monomer with the use of the inhibitor, N-hydroxy-N-methylglucamine, at a level of 0.03 to 0.5 wt. % of the aqueous phase. A typical manufacturing cycle between kettle cleanings is five to seven batches with the styrene-divinyl-benzene monomer system.
The free radical quenching agent or moiety of the present invention can be N-nitrosoamines and N-hydroxylamines. The hydrophilic tail can be polyhydric tail and the polyhydric tail can comprises from about three carbon atoms to about one thousand carbon atoms, each of the carbon atoms can be attached to a hydroxyl group except the carbon atom attached to the nitrogen of the group. The hydrophilic tails can also constructed from sugar molecules, dextrans, mositol, sorbitol, poly(vinyl-alcohol)s, poly(ethylene oxide)s, aminoacids, oligomers of N-vinylpyrrolidinone, oligomers of N-vinylformamide, oligomers of acrylamide, oligomers of methacrylamide, p-benzamide, p-benzene sulfonamide, etc.
In a further embodiment, the present invention also relates to a method for quenching of radical polymerization in the aqueous phase. The method comprises the step of providing a free radical quenching agent or moiety having a hydrophilic tail. The agent can be N-hydroxylamine or N-nitrosoamine. The hydrophilic tail can be a polyhydric alcohol tail. The polyhydric alcohol tail can comprise from about three carbon atoms to about one thousand carbon atoms, each of the carbon atoms being attached to a hydroxyl group except the carbon atom attached to the nitrogen of the N-hydroxylamine or the N-mitrosoamine. The hydrophilic tail can also be selected from sugar molecules, dextrans, insotol, sorbitol, polyvinylalcohols, polyethylene oxides, amino acids, oligomers of N-vinylpyrrolidinone, oligomers of N-vinylformamide, oligomers of acrylamide, oligomers of methacrylamide, p-benzamide and p-benzenesulfonamide.
In one embodiment, the inhibitors is N-hydroxyl-N-methyglylamine and in another embodiment, the inhibitor is N-nitroso-N-methyglucamine. In a further embodiment, the method further comprises the step of dissolving the agent in a liquid medium such as water, alcohol and mixtures thereof.
In still another embodiment, the present invention also relates to a method for coating a surface against polymer growth. The method comprises the step of applying a free radical quenching agent or moiety having a hydrophilic tail to a surface. The surface can be the walls of a reactor and the baffles, blades, and shaft of the agitator. The agent or moiety can be N-hydroxylamine or N-nicrosoamine. The polyhydric alcohol tail comprises from about three carbon atoms to about one thousand carbon atoms, each of the carbon atoms being attached to a hydroxyl group except the carbon atom attached the nitrogen of the N-hydroxylamine or the N-nitrosoanime. The hydrophilic tail consisting essentially of sugar molecules, dextrans, mositol, sorbitol, polyvinylalcohols, polyethylene oxides, amino acids, oligomers of N-vinylpyrrolidinone, oligomers of N-vinylformamide, oligomers of acrylamide, oligomers of methacrylamide, p-benzamide and p-benzenesulfonamide. In one embodiment, the method further comprises the step of dissolving the agent in a liquid medium before applying the agent to the surface. The liquid medium consist essentially of water, alcohol and mixtures thereof the agent can be applied to the surface by spraying, brushing and other numerous application methods.
The inhibitor of the present invention exhibits many advantageous properties including the following properties: is immiscible or insoluble within the droplets of monomer; is miscible or soluble in the aqueous phase; is effective at quenching free radicals that arise in the aqueous phase; remains in the aqueous phase throughout the transformation of the monomer droplets into polymeric particles; and the quenching lifetime for the inhibitor lasts essentially throughout the time needed to polymerize the monomer droplets.
The following examples which describe the method of synthesis of aqueousphase free radical quenching agents for use in suspension polymerization are illustrative of the invention in more detail and are not intended to be construed in anyway as limitations to the invention.